Numerous types of machine tool control systems exist in the prior art. Generally, in such systems, the motions of the machine tool parts are controlled in accordance with predetermined programs corresponding to the various movable machine tool parts. The actual position or orientation of a movable machine tool component with respect to a particular axis, is determined by a position setting device associated with that axis. In a closed loop control system, the desired position or orientation of a machine tool component, and the actual position or orientation thereof with respect to a particular axis, are fed to at least one position controller which generates a control signal encoding the value of a desired rate of translation or rotation, this control signal being transmitted to the drive associated with the respective axis.
Such systems may be configured as point to point, or master-slave axis arrangement, or time base axis arrangement. In the point to point control system, possibly the most common machine control implementation, each axis is given a destination and when all the axis have arrived at their destination, a new set of destinations are then issued. The master-slave and the time base arrangement, have the advantage over the point to point control of not being required to wait for discrete synchronization points before updating. In the master-slave system, the slave axis tracks the master axis which travels at a predetermined velocity. Therefore, if the master axis runs too fast, then the slave axis will lag behind, as a result the slave axis velocity must be increased to compensate for the lag. In general, with such a system, any perturbation in the master axis will be reflected in the slave axis, because the slave axis is tracking the master axis.
In all the control arrangements, a tradeoff is made between speed and accuracy. The greater the distance traversed without a correction, the greater will be the error. This error can be reduced by reducing speed, thereby reducing the distance traversed between corrections. For example, if a design required an error correction every quarter micrometer, and the system update time was ten milliseconds, then the maximum feed rate would be 0.025 millimeters per second, or approximately seventeen minutes per inch.
It is, therefore, an aspect of the instant invention to provide a machine control system which provides greater precision and accuracy at higher speeds than prior art machine control systems.
In many prior art machine control systems, the processor for these controllers is designed to rapidly execute programmable controller type instructions, which in medium to large sized controllers include not only instructions that manipulate single-bit input and output data, but also arithmetic instructions, file handling instructions, timers and counters, sequencer and other more complex instructions. To insure that the programmable controller can respond quickly to change in the status of sensing devices on the controlled system, it is imperative that the controller execute the control program repeatedly at a very high rate. The rate at which a programmable controller can execute the instructions, the type of instructions, as well as the size of the control program, are the primary factors which determine the rate at which the programmable controller can repeatedly execute, or "scan", the control program.
A problem arises, however, with machine control systems that require a high degree of accuracy and precision, while maintaining a moderate cutting speed. That is, the processor driving the controller must be able to respond very rapidly in order to achieve great precision at speed, an objective that many prior art controllers are unable to meet.
It is, therefore, another aspect of the instant invention to provide a machine control system which utilizes a processor configuration capable of rapidly handling large amounts of data to insure precision, accuracy, and speed.